I had the same problem but I was successful importing from an self produced xml (for the postmeta).
<wp:postmeta>
<wp:meta_key>telefono</wp:meta_key>
<wp:meta_value><![CDATA[$tel_azienda]]></wp:meta_value>
</wp:postmeta>
The problem was that after that the data were not available to use with MF, I’ve solved it with this mysql query:
INSERT IGNORE wp_mf_post_meta (`id`, `group_count`, `field_count`, `post_id`, `field_name`, `order_id`)
SELECT `meta_id`, 1, 1, `post_id`, `meta_key`, 1 FROM `wp_postmeta` WHERE `meta_key` = 'mini_descrizione' OR `meta_key` = 'imagen_en_categoria' OR `meta_key` = 'direccion' OR `meta_key` = 'codigo_postal' OR `meta_key` = 'ciudad' OR `meta_key` = 'provincia' OR `meta_key` = 'telefono' OR `meta_key` = 'email'OR `meta_key` = 'web' OR `meta_key` = 'google_map'
It copies the metas from the wp_postmeta table to the wp_mf_post_meta.